This invention relates to the use of conductive nitrides, particularly TiN, as electrode material in organic light emitting devices.
Organic light emitting devices (OLEDs), which make use of thin film materials that emit light when excited by electric current, are becoming an increasingly popular form of flat panel display technology for applications such as television sets, computer terminals, telecommunications equipment and a host of other applications. There are presently three predominant types of OLED construction: the xe2x80x9cdouble heterostructurexe2x80x9d (DH) OLED, the xe2x80x9csingle heterostructurexe2x80x9d (SH) OLED, and the single layer polymer OLED.
In the DH OLED, as shown in FIG. 1A, a substrate layer of glass 10 is coated by a thin layer of indium-tin-oxide (ITO) 11. Next, a thin (100-500 xc3x85) organic hole transporting layer (HTL) 12 is deposited on ITO layer 11. Deposited on the surface of HTL 12 is a thin (typically, 50xc3x85-500xc3x85) emission layer (EL) 13. The EL 13 provides the site for electrons injected from a 100-500xc3x85 thick electron transporting layer 14 (ETL) to recombine with holes from the HTL 12. Examples of prior art ETL, EL and HTL materials are disclosed in U.S. Pat. No. 5,294,870, the disclosure of which is incorporated herein by reference.
Often, the EL 13 is doped with a highly fluorescent dye to tune color and increase the electroluminescent efficiency of the OLED. The device as shown in FIG. 1A is completed by depositing metal contacts 15, 16 and top electrode 17. Contacts 15 and 16 are typically fabricated from indium or Ti/Pt/Au. Electrode 17 is often a dual layer structure consisting of an alloy such as Mg/Ag 17xe2x80x2 directly contacting the organic ETL 14, and a thick, high work function metal layer 17xe2x80x3 such as gold (Au) or silver (Ag) on the Mg/Ag. The thick metal 17xe2x80x3 is opaque. When proper bias voltage is applied between top electrode 17 and contacts 15 and 16, light emission occurs from emissive layer 13 through the glass substrate 10. An LED device of FIG. 1A typically has luminescent external quantum efficiencies of from 0.05% to 2% depending on the color of emission and the device structure.
The (SH) OLED, as shown in FIG. 1B, makes use of multi-functional layer 13xe2x80x2 to serve as both EL and ETL. One limitation of the device of FIG. 1B is that the multi-functional layer 13xe2x80x2 must have good electron transport capability.
A single layer polymer OLED is shown in FIG. 1C. As shown, this device includes a glass substrate 1 coated by a thin ITO layer 3. A thin organic layer 5 of spin-coated polymer, for example, is formed over ITO layer 3, and provides all of the functions of the HTL, ETL, and EL layers of the previously described devices. A metal electrode layer 6 is formed over organic layer 5. The metal is typically Mg or other conventionally used low work function metal.
The choice of materials to be used in OLEDs is based on several criteria. For example, the anode in a conventional OLED must have good optical transparency, good electrical conductivity and chemical stability. Indium tin oxide (ITO) meets these criteria and is the most widely used anode material in OLEDs. ITO films combine high transparency (≈90%) with low resistivity (1xc3x9710xe2x88x923-7xc3x9710xe2x88x925xcexa9xc2x7cm) and can be prepared by a variety of methods including sputtering, chemical vapor deposition (CVD) and sol-gel techniques.
However, OLEDs using ITO films do have a few areas which could be improved upon. First, the work function of ITO (4.4-4.7 eV, based on ultraviolet photoemission spectroscopy measurements) lies near the HOMO levels of typical OLED hole transporting or injecting materials, thus leading to a barrier for hole injection into organic material. Second, an OLED""s stability and efficiency strongly depend on the nature of the anode/organic film interface. Therefore, any changes in this interface over time will destabilize the OLED. For example, one cause of long term OLED degradation involves the diffusion of metal ions or oxygen from the ITO film into the organic film. Finally, another issue arising from the use of an ITO anode is the tendency of SnOx and InOx islands to form through reorganization of the ITO film over time.
Therefore, although ITO electrodes have been used with many different organic materials, additional OLED anode materials are needed.
The present invention provides organic light emitting devices, including polymer, (e.g. single and multi-layer), single heterostructure, and double heterostructure, which use conductive nitride films as anode material. These anode layers can be transparent or opaque and can be used in OLEDs with transparent cathodes. In addition, the formed OLEDs with TiN or TiN/ITO anode layers can be used to form stacked OLEDs.
The OLEDs of the present invention can be incorporated in electronic devices, including computers, monitors, televisions, large area wall screens, theater screens, stadium screens, billboards, signs, vehicles, printers, telecommunication devices, and telephones.
One embodiment of the present invention comprises forming a thin TiN film on a glass substrate and then forming an OLED using the TiN film as an anode layer. Another embodiment of the present invention uses a multi-layered anode of TiN on top of ITO in forming an OLED.